1. Field of the Invention
The present invention relates to a flat non-aqueous electrolyte secondary cell and in particular to a flat non-aqueous electrolyte secondary cell with improvements in heavy loading discharge characteristics.
2. Description of the Prior Art
In recent years, there are commercially available coin- or button-shaped flat non-aqueous electrolyte secondary cells wherein metal oxides such as MnO2 and V2O5, inorganic compounds such as fluorinated graphite, or organic compounds such as polyanline and polyacene structural compounds are used as the cathode active material, while metal lithium or lithium alloys, organic compounds such as polyacene structural compounds, carbon materials capable of occluding and releasing lithium, or oxides such as lithium titanate or lithium-containing silicon oxides are used in the anode, and non-aqueous electrolytes containing a supporting electrolyte such as LiClO4, LiPF6, LiBF4, LiCF3SO3, LiN(CF3SO2)2 and LiN(C2F5SO2)2 dissolved in a non-aqueous solvent such as propylene carbonate, ethylene carbonate, butylene carbonate, diethyl carbonate, dimethyl carbonate, methyl ethyl carbonate, dimethoxyethane and γ-butyl lactone are used as the electrolyte. These cells are used as power sources for backing up SRAM and RTC where an electric current is discharged for light loading of about several to dozens μA, or as main power sources for wristwatches not requiring cell exchange.
In general, these coin- or button-shaped flat non-aqueous electrolyte secondary cells have the structure shown in FIG. 4. That is, a metallic anode case 5 also serving as an anode terminal and a metallic cathode case 1 also serving as a cathode terminal are fit to each other via an insulating gasket 6, and further the cathode case 1 has a sealed opening structure caulked by caulking, and in the inside of this structure, tablet-shaped cathode 12 and anode 14 having a smaller diameter than the opening of the insulating gasket 6 are set up against each other via a single- or multi-ply separator 13 impregnated with a non-aqueous electrolyte.
The coin- or button-shaped flat non-aqueous electrolyte secondary cells as described above have the advantage that they are easily producible, excellent in mass-productivity, and superior in long-term reliability and safety. Further, by virtue of their simple structure, the most distinctive feature of these cells is that their miniaturization is feasible.
Meanwhile, the miniaturization of devices (mainly compact information terminals) such as portable telephones and PDA is promoted, thus making it essential to miniaturize secondary cells as their main power sources. In these power sources, there have been used cylindrical or rectangular alkali secondary cells such as lithium ion secondary cells wherein lithium-containing oxides such as lithium cobaltate is used as the cathode active material while a carbon material is used in the anode, or nickel hydride secondary cells wherein nickel oxyhydroxide is used as the cathode active material and a hydrogen-occluding alloy is used as the anode active material. These cells have been constructed by coating or filling a current-collecting body consisting of a metal foil or metal net with an active material layer to form an electrode, then welding a tab terminal into the center of the electrode, and winding or laminating it to form an electrode group, complicatedly bending the tab terminal from the center of the electrode group and welding the terminal into a safety element, an opening-sealed pin or a cell can. However, these cells have been constructed in such a complicated process that they are inferior in workability and the miniaturization of parts therein is also difficult. Further, these cells should be provided therein with a space for preventing the tab terminal from short-circuiting or for integrating a large number of parts such as safety element into the cells, and thus there is a limit to the miniaturization of these cells at present.
For miniaturization of the cells under these circumstances, the present inventors have attempted not at miniaturizing cylindrical or rectangular lithium ion secondary cells or nickel hydride secondary cells, but at achieving a higher output of the flat non-aqueous electrolyte secondary cells described above. That is, the present inventors have used lithium cobaltate of high capacity and high potential as the cathode active material and a graphitized carbon material of high capacity excellent in voltage evenness as the anode active material, and according to the process and structure of the conventional flat non-aqueous electrolyte secondary cell, the inventors have processed the cathode and anode into tablets smaller than a gasket, to prepare a cell.
However, this cell though attaining superior characteristics to the conventional flat non-aqueous electrolyte secondary cell is not satisfactory when discharged in a large current required of a main power source in compact portable devices, thus failing to achieve levels satisfactory as a main power source in compact portable devices. Accordingly, the development of techniques for permitting the heavy-loading discharge characteristics of the compact flat non-aqueous secondary cell to reach levels not achieved in the prior art is necessary.